The present invention generally pertains to a device and method for evaluating and reporting motor condition and more particularly, to an improved method and apparatus for assessing stator condition while the motor is operating.
Motors, particularly electrical motors, play a key role in industry. Such motors are used to drive fans, pumps, compressors, valves, and many other machines. It is potentially very costly to allow a significant problem to go on unnoticed in either the motor or the motor driven machine. It is also costly and very time consuming to attempt to repair a nonexistent problem. With present methods of analyzing motor performance, these costly situations often occur. Thus there is a need for an improved diagnostic method and apparatus for use with motors and motor driven machines.
Most prior art motor monitors for detecting winding faults require the use of coils intrusively placed within the motor, or alternately, monitoring the neutral return line for radio frequency evidence of arcing. These monitors require extra hardware beyond the normally used three current and three voltage probes. Additionally, in most motors the neutral wire is unavailable. Accordingly, to implement comprehensive detection, all three electrical leads have to be monitored with high frequency probes in addition to the usual power frequency probes. This involves multiple additional sensors and channels, high frequency sensors, and extra signal conditioning.
A more recent innovation (U.S. Pat. No. 5,270,640, Kohler et al, Dec. 1993) calculates the effective negative sequence resistance (R.sub.2) and reactance (X.sub.2) from the negative sequence voltage and current vectors, and tracks these parameters for indications of winding degradation. The present invention also uses the negative sequence resistance and reactance to monitor deterioration in motors, but calculates these parameters directly from the normal, as opposed to sequence, voltage and current vectors thus avoiding two coordinate transformations. The present invention further provides a means for calibrating the current probes, and adjusting the algorithm used for calculating the negative sequence parameters in the event the normal multi-phase sequence (e.g., ABC) is reversed (CBA). Further, the present invention provides a means for establishing thresholds for determining when the motor winding becomes non-normal.
A memory is included for storing motor data (e.g., in a database), which allows for comparisons to be made between the motor's present condition and performance, historical data for the same motor to determine the presence of trends, and data on similar "sister" motors to ascertain normality with respect to a population of similar motors. The database accumulates knowledge about the motor, which may be sorted by motor load. The information provided by the present invention is useful for changing motor operating load, scheduling maintenance on the motor, planning for replacement of the motor, or even shutting the motor down, as is appropriate. Furthermore, the system compares multiple types of motors under similar operating conditions so that comparisons can be made, which are useful, for instance, in making purchasing decisions.
The present invention can be used to provide either on-line continuous monitoring of motor condition, or as a periodic inspection tool. The present invention also provides detailed analysis capability for intensive diagnostic work and a database for determining trends and relative performance (compared to similar motors). The present invention avoids the intrusive measurement problems associated with most prior art, and avoids the complex intermediate vector calculations required in the more recent prior art, U.S. Pat. No. 5,270,640. In addition, the present invention corrects for phase sequence reversal, offers diagnostics based on "sister" motor data as well as historical data, and categorizes the data by load to provide for an optimum baseline.